Read 'input', apply quoted-printable encoding, and write to 'output'.
'input' and 'output' are files with readline() and write() methods.
The 'quotetabs' flag indicates whether embedded tabs and spaces should be
quoted. Note that line-ending tabs and spaces are always encoded, as per
RFC 1521.
The 'header' flag indicates whether we are encoding spaces as _ as per
RFC 1522.

Decide whether a particular byte ordinal needs to be quoted.
The 'quotetabs' flag indicates whether embedded tabs and spaces should be
quoted. Note that line-ending tabs and spaces are always encoded, as per
RFC 1521.